1. Field of the Invention
The present invention relates generally to a system for amplifying and reradiating received radio waves and more particularly to a "high-frequency signal booster" or "booster amplifier" for amplifying and reradiating high-frequency waves to a dead area of a small-cell mobile radio communication system (for example, a cell enhancer for amplifying and reradiating radio waves to a cell-drop area of a cellular mobile telephone system) and for expanding a "service area" of a base station to the outside of an existing "service area". The term "service area" is used in this application to describe the area which is effectively served by a base station, that is, the coverage of the base station. Further, the term "service territory" is used to describe the area or territory in which a base station has the right to supply communication service to a mobile unit being moving therein and charge the owner of the mobile unit for the service.
2. Background Discussion and Objects of the Invention
Directing now attention to FIG. 1, there is shown a conventional high-frequency (HF) signal booster. In this figure, reference numerals 1, 2, 3 and 4 denote a "base station antenna" for use in communicating with a base station, a "mobile station antenna" for use in communicating with a mobile station, an antenna duplexer constituting a "base station antenna" system, and another antenna duplexer constituting a "mobile station antenna" system, respectively. Further, reference numerals 5 and 6 denote linear power amplifiers.
Next, operation of this conventional HF signal booster will be described hereinbelow.
First, radio waves are received by the base station antenna 1. Then, the incoming signal is led through the antenna duplexer 3 to the power amplifier 5. The thus amplified signal is transmitted to the mobile station antenna 2 through the antenna duplexer 4 and further radio waves are radiated therefrom into space.
On the other hand, in case of radio waves received by the mobile station antenna 2, the incoming signal is led through the antenna duplexer 4 to the power amplifier 6. The amplified signal is further transmitted to the base station antenna 1 through the antenna duplexer 3 and radio waves are radiated therefrom into space.
The above-described conventional system is attended with the following problems. A frequency of an incoming or input signal is equal to that of an output signal. Error rate of digital signals increases and additionally interference noises occur due to mutual interference between the radio wave which is radiated from the base station and then directly received by the booster and that which is amplified by the booster and then reradiated.
Accordingly, it is an object of the present invention which is made in order to obviate the above-described problems to provide an HF signal booster being capable of varying the frequencies of the input and output signals and effecting modulation of the output signal by employing a signal of which frequency is in the voice band (hereunder referred to simply as an "out-of-voice-band signal").
To accomplish this object, in accordance with a first aspect of the present invention, there is provided an HF signal booster including a power amplifier which can be used in common in both operations of amplifying input signals sent from a base station and of amplifying signals sent from a mobile station (hereunder referred to simply as "up and downstream operations").
Another example of the prior art system of this type is described in Paper Collection Vol. 37 of the Study Publication Association of the Electric Communication Institute (Nippon Telegraph & Telephone Public Corporation).
Referring to FIG. 2, there is shown the conventional booster of this type. As in FIG. 1, reference numerals 1 and 2 indicate a receiving antenna and a transmitting antenna, respectively. Further, reference numeral 23 denotes a band-pass filter/pre-amplifier; 24 a down-converter; 25 an intermediate-frequency filter/amplifier; 26 an up-converter; 27 a power amplifier/band-pass filter; 28 a highly stable crystal oscillator; and 29 a station-originating signal generator.
Further, operation of this conventional system will be explained hereinbelow.
High-frequency signals received by the receiving antenna 1 are selectively amplified by the band-pass filter/pre-amplifier 23 and applied to the down-converter 24, in which the received signals are combined with signals supplied from the station-originating signal generator 29 and are thereby converted into intermediate-frequency signals. The thus converted signals are further selectively amplified by the intermediate-frequency filter/amplifier 25 and applied to the up-converter 26. The intermediate-frequency signals are combined with the signals fed from the station-originating signal generator 29 in the up-converter 26, whereby they are converted into high-frequency signals. The high-frequency signals are selectively amplified by the power amplifier/band-pass filter 27 and are then transmitted from the transmitting antenna 2. The station-originating signal generator 29 is controlled by the highly stable crystal oscillator 28. The received signals and the transmitted signals can be held at the same frequency by supplying signals of the same frequency to the down-converter 24 and the up-converter 26.
The conventional system is constructed in the above-described manner. Consequently, the signals given forth from the transmitting antenna are received by the receiving antenna once again and thus what is called a `singing` phenomenon, that is, self-oscillation takes place. Prevention of the occurrence of this phenomenon requires such an arrangement that a coupling loss or attenuation occurring between the transmitting and receiving antennas is set to a value sufficiently greater than a gain of the booster. An additional defect of the prior art system is that the occurrence of the `singing` phenomenon cannot be automatically detected from variations in condition of the system. The present invention is accomplished to obviate these problems of the conventional system.
Accordingly, another object of the present invention is to prevent the `singing` phenomenon by automatically controlling the gain of the booster when the coupling loss occurring between the transmitting and receiving antennas decreases for some reason and to further permit giving an alarm through a detection circuit.
FIG. 3 is a block diagram illustrating a still another conventional system. In this figure, reference numeral 1 denotes a "base station antenna"; 2 a "mobile station antenna"; 3 and 4 antenna duplexers; 39 and 321 one-to-two branching devices; 310a and 322a elements each having "narrow-band selectivity" (that is, passing a band of frequencies which has relatively small bandwidth); 310b and 322b elements each having "wide-band selectivity" (that is, passing a band of frequencies which has relatively large bandwidth); 311 and 323 synthesizers (or combiners); and 312 and 324 wideband power amplifiers.
Next, operation of this conventional system will be described hereinbelow. Radio waves from the base station are received by the base station antenna 1 and the input signals are led through the antenna duplexer 3 to the branching device 321 where they are branched and further led to the elements 322a and 322b. The element 322a having the narrow-band selectivity is allocated for a control channel. On the other hand, the element 322b having the wide-band selectivity is allocated for a message channel. The radio waves have been selectively amplified by these elements 322a and 322b. Thereafter, the amplified waves are synthesized (or combined) by a dual synthesizer 323 and further amplified by a common amplifier 324. Furthermore, the synthesized and amplified waves are reradiated from the mobile station antenna 2 through the antenna duplexer 4. Thus, the radio waves received by the base station antenna 1 are reradiated at the same frequency from the mobile station antenna 2.
On the other hand, the radio waves transmitted from the mobile station are received by the mobile station antenna 2 and branched off through the antenna duplexer 4. The received radio waves are further transmitted through a route which is similar to the above-described one, (that is, by way of the branching device 329, the element 310a having the narrow-band selectivity, the element 310b having the wide-band selectivity, the dual synthesizer 311, the common amplifier 312 and the antenna duplexer 3) to the base station antenna 1 from which they are reradiated.
Since this conventional HF signal booster has the foregoing construction, it follows that a plurality of radio waves are applied to the common amplifiers 312 and 324. This results in defects of this conventional system that an output power having a large magnitude cannot be obtained for retraining unnecessary waves produced by inter-modulation and that in the control channel, there increases a bit error rate of digital signals due to mutual interference between the radio wave directly coming from the base station (that is, the direct signals) and the wave reradiated from an HF signal booster. The present invention is accomplished to eliminate these problems of the conventional system.
Accordingly, it is a still further object of the present invention to provide an HF signal booster being capable of obtaining the large output power even when restraining the inter-modulation.
Yet another object of the invention is to prevent the mutual interference between the reradiated waves and the directly coming waves.
Turning now attention to FIG. 4, there is illustrated an example wherein two service areas for an existing cellular mobile telephone system and a booster system are adjacent to each other. Reference numerals 43 and 44 designate base stations. Further, reference numerals 41 denotes a transmitting antenna of the base station 43. Moreover, reference numerals 42 and 45 denote a mobile station antenna and a base station antenna of the booster station 44, respectively. Reference numeral 46 indicates a curve showing an intensity of the radio wave emitted from the transmitting antenna 41 of the base station 43; and 47 a curve showing an intensity of the radio wave emitted from the base station antenna 42 of the booster station 44. Further, reference numeral 48 indicates a direction of transmission of the RF signal, which is transmitted from the antenna 41 of the base station 43 and received by the antenna 45 of the booster station 44. Moreover, reference numeral 49 indicates a borderline drawn through a spot at which the intensities of radio waves transmitted from the adjacent transmitting antennas 41 and 42 are substantially equal with each other.
Next, operation of this conventional system will be described hereinbelow. The mobile station is generally adapted to scan control channels or set-up channels constantly assigned when the mobile station is located in an intermediate zone in which the contiguous service areas are partly overlapped with each other. The mobile station is also adapted to choose the strongest channel to be used for transmitting and receiving messages.
For instance, when the mobile station is positioned in a region lying between the base station 43 and the borderline 49, the mobile station accesses the station 43. Contrarily, when the mobile station is positioned in another region lying between the booster station 44 and the borderline 49, the mobile station accesses this station booster 44. If the mobile station is positioned around the borderline 49, interference between the direct signals from the transmitting antenna 41 of the base station 43 and the reradiated signals from the mobile station antenna 42 of the booster station 44 is observed due to the time lag between the reception of the direct signals and the reradiation of the signals. Thereby, error detection of the digital data transmission can be effected. The present invention is accomplished to eliminate these problems of the conventional system.